Diabetic Retinopathy (DR) is an eye disease affecting millions of patients with type I and type II diabetes. 45% of Americans diagnosed with diabetes have some form of DR, and nearly all patients who have type I diabetes for more than 20 years will show signs of DR. In the US, DR is responsible for almost 8% of legal blindness and is the leading cause of new cases of blindness in adults of 20-74 years of age. In addition, the presence of DR is growing at a dramatic rate in developing countries as the number of type II diabetics increases. In India, diabetic retinopathy has jumped from 17th to the sixth leading cause of blindness within twenty years. Similar increases can be seen worldwide and has become a key concern for the World Health Organization.
With close monitoring and annual eye exams, diabetic retinopathy can be diagnosed and successfully treated. According to the American Academy of Ophthalmology (AAO), 95 percent of diabetics with DR can avoid vision loss if treated on time. For this reason, a yearly dilated screening exam is currently recommended by the AAO for all diabetic patients. Traditionally dilated screening exams have required referral to eye care specialists because patient's primary care facilities lack the necessary tools and staff to properly diagnose DR. Due to the inconvenience and extra cost associated with seeing a specialist, patients often fail to follow up on their referral.
Over the last two decades, multiple attempts have been made to address these barriers to screening involving placement of a camera for retina photography within the primary care physician's office. These efforts have largely been hampered by difficult to use and expensive retinal diagnostic devices costing upward of $50,000. Retina camera cost has remained high due to the inherently demanding optical and illumination design that requires complex manufacturing processes. Usability and cost of the devices have similarly been limited by the need to implement a custom image recording device to accurately record images formed by these complex optical designs. These custom image recording devices usually require multiple image sensors and image display units for initial image alignment/focusing and actual final image acquisition. Further, they often require manual control of focusing, image exposure, and white balance, to achieve acceptable final image acquisition, limiting their use to skilled practitioners of ophthalmic photography.
A standard consumer camera device such as a “point and shoot” or DSLR camera does not have an inherent capability to perform fundus photography given the complex optics and illumination required to form an image of the retina. In most existing fundus cameras, the device forms a donut of illumination measuring approximately 3 mm to 7 mm that is focused on the patient's eye. This donut of light is formed by the fundus camera itself through a series of optics and built in illumination source. Alternatively, off axis non-donut illumination can be used and is the basis of indirect ophthalmoscopy. This approach is not commonly employed commercially as photographic quality is impacted negatively by this technique, in particular with respect to evenness of the field illumination as compared to donut illumination. Condensing lenses are required to form an image of the retina that is created by the reflection of light off the back of the eye in response to the donut illumination. Most cameras then employ a separate optics stage to focus the retinal image produced by the condensing lens onto a CCD or CMOS image sensor either built into or externally attached to the device (e.g., a consumer camera device digital camera back which consists of the DSLR body with image sensor alone without the detachable front DSLR camera lens on it). Even when a digital camera back is used, this approach has significant limitations in that auto-focus, auto-exposure, auto-white balance capabilities of the consumer camera device are no longer available. Finally, non-mydriatic fundus photography, in which the patient does not require prior pharmacologic dilation, is commonly used in screening photography and relies on infrared wavelength light to compose and focus the image of the retina before final image capture. To achieve proper color balance on the final image recorded on the camera, many devices split off the infrared wavelengths to a second infrared sensitive image sensor to be used for focusing, and send the visible wavelength light to a separate visible light sensitive image sensor for final image capture. When a consumer camera device digital camera back is used for final image capture, this split off is required as consumer camera devices are sensitive only to visible light wavelengths.
Conventional fundus cameras require manual control of image exposure and flash power resulting in the need for multiple photos to obtain a correctly exposed image.
Many conventional fundus cameras are heavy and large and require external power and observation monitors and other systems, which add to their size and limit their mobility. True size portability and battery based power is critical for establishing mobile screening clinics where independent traveling clinicians, for example, must be able to easily set up low-cost screening programs in areas with little access to electricity and other medical equipment.
The complexity of conventional systems requires extensive training and the need for specialists to obtain adequate images of the fundus. This limits the ability to deploy fundus cameras to a large number of clinics, especially primary care clinics, and therefore prevents those who may need treatment from getting screened for retinopathy. Furthermore, many conventional fundus cameras are expensive and cannot be produced at a low cost. This also prevents deployment of the diagnostic imaging equipment necessary to screen many patients.
For non-mydriatic diagnostics, existing fundus cameras generally require the use of two separate systems for observation and photography of the fundus. In these systems an infrared imaging sensor is needed for observation and another imaging sensor is needed for visible light photography. This is particularly true when a consumer camera device body is used to record images as these devices are only sensitive to visible wavelengths of light. This has the disadvantage of adding cost, complexity in manufacture, and size to the overall fundus camera design and prevents these cameras from achieving true mobility and portability at a low cost.
CCD and CMOS sensors used in consumer camera devices are inherently sensitive to infrared wavelengths and an infrared cutoff filter is used to prevent infrared wavelengths from reaching the image sensor of the consumer camera device. If this filter were not present, significant image distortion from infrared wavelengths would be present in the recorded image. Some consumer camera devices have used a single sensor to record both infrared and visible wavelengths of light by using a mechanical filter that is automatically rotated into position and filters out infrared wavelengths when taking a normal visible spectrum photo. This is an unnecessarily complex and difficult to manufacture mechanism and this design is no longer in commercial production.
While conventional approaches discussed above may provide certain capabilities, none of their uses achieve true portability, in an inexpensive, pragmatic form so as to solve problems as described above.